Simon set up Walk-In Media in October 2019, in partnership with MSQ. He had previously been the CEO of Publicis' media agency Blue 449 (now Spark Foundry). He founded Walk-In Media with the aim of giving all advertisers the clear benefits of an independent agency, but with all the specialist capabilities of the network agencies.

Walk-In Media merged with agenda21 (another MSQ agency) in September 2020.